The conventional method for producing soluble coffee products is described in M. Sivetz, "Coffee Technology," wherein roasted and ground coffee is extracted with heated water by the countercurrent method in a percolator set usually consisting of five to eight extraction columns connected in series, and the resulting coffee extract is subjected to drying and other after-treatments. A cycle of extraction in this method starts at the extraction column filled with the most spent roasted and ground (R&G) coffee and ends at the column filled with the freshest coffee. Therefore, in the fresher stage column, the coffee solids are extracted with a liquid that has a certain concentration of soluble coffee solids. As noted by Sivetz, the initial stage of extraction is performed by osmosis, so the efficiency of extraction depends on the difference between the solids content of the coffee cells and the concentration of the liquor to be extracted. Since the extraction liquid usually contains a larger amount of soluble solids, it is readily understood that the efficiency of extraction is decreased toward the progressively fresher stage columns.
As is well known, the aromatic components of coffee are susceptible to heat damage and are desirably extracted and processed at low temperatures. In other words, the desired coffee extract is obtained with high efficiency by extracting a fresh column in the initial stage with a liquor having a minimum content of soluble coffee solids.
To achieve this object, the prior art has relied on split extraction using a percolator set generally consisting of five to eight extraction columns. Split extraction consists of primary extraction wherein a column filled with freshly roasted and ground coffee and one or two subsequent columns are extracted with fresh water initially containing no soluble solids, and secondary extraction wherein the columns that have gone through the primary extraction step are subjected to secondary countercurrent extraction by feeding the most spent column with heated water. In the primary extraction, the fresher column is extracted at a relatively low temperature (e.g., 80.degree.-150.degree. C.) to minimize the damage to the aromatic components by heat, whereas in the secondary extraction, most of the columns are fed with high-temperature water (e.g., 160.degree.-90.degree. C). Therefore, the split extraction uses two, rather than one, water lines.
The advantages of split extraction have been recognized in many prior patents (e.g., U.S. Pat. Nos. 3,790,689, 3,810,766 and 3,965,269) and coffee makers have made efforts to use the process on a commercial scale. But their efforts have been hampered by the difficulty of achieving consistent operation in the overall process due mainly to the low concentration of draw obtained in the initial stage of the secondary extraction. Since the fresher column is desirably extracted at low temperatures, the extract obtained in the later stage of the primary extraction generally has a low concentration of soluble coffee solids. The effluent initially obtained in the secondary extraction is mainly composed of the residue from the primary extraction. To eliminate any void in an extraction column filled with roasted and ground coffee, water weighing about 1.5 to 2.0 times as much as the coffee is generally necessary. In other words, a true secondary extraction cannot start in the column unless water weighing about 1.5 to 2.0 times as much as the coffee is "pushed out" of it. The concentration of the secondary extract including the " pushed out" initial draw varies with the volume of the liquor to be extracted, but is generally in the range of from about 3 to 8% by weight coffee solids. In order to increase this concentration to the generally desired level, 80 to 90% of the water must be removed. To remove that much water by, say, thermal concentration, at least 8 to 10 kcal of heat is necessary per gram of soluble coffee solids, and the coffee solids undergo "heat damage" that adversely affects the quality of the final product. U.S. Pat. No. 3,810,766 to Holzberg recycles secondary extract the secondary extraction columns to increase the concentration of the secondary extract. Nonetheless, it is apparent that an advance in the art is needed to more effectively extract the coffee solids and preserve the quality of the coffee solids to the final product.